In a small saucepan, combine mirin, miso, and sugar. Whisking constantly, cook over medium heat until sugar has dissolved. Remove from heat, and let cool completely.
Heat broiler, with rack 6 inches from heat. Coat a baking sheet with oil. Place fish on baking sheet, and brush liberally with miso mixture. Broil until fillets are browned on top and opaque in the center, 6 to 8 minutes. If tops brown before fish is cooked through, cover loosely with aluminum foil. Remove from oven; serve immediately.